Process for the foaming of acyloxysilane-containing silicone masses

ABSTRACT

The present invention pertains to a process for foaming mixtures of bifunctionally-ending di-organopolysiloxanes and acyloxsilane cross-linking agents, involving the addition to the mixture of sufficient quantities of an ammonium, amino, sodium or potassiumn hydrocarbonate.

The present invention concerns a process for the foaming of mixtures ofat least bifunctionally-terminated diorganopolysiloxanes, acyloxysilanecross-linking agents, as well as possibly filling materials.

Such organopolysiloxane mixtures, also known as cold-vulcanising,monocomponent silicone rubbers, which possibly also contain suitableadditives, pigments, colouring materials, oxidation-, heat- andlight-protective pigments, as well as solvents and plasticisers, and, ina state ready for working up, are present in liquid or pasty form, aredescribed as such in FR 1 198 749 or U.S. Pat. No. 3,133,891. Themixtures usually cross-link at room temperature with the take up ofwater from the surrounding atmosphere to give rubber-elastic polymers.As cross-linkers, there are used tri and higher functional acyloxysilanecompounds which, by reaction with the polysiloxane or by hydrolysis,split off carboxylic acids and thus initiate the formation of amacromolecular meshwork. After hardening out has taken place, suchmasses are characterised by a good inherent adhesion to the most variedmaterial surfaces and by a generally high stability against the actionof temperature, light, moisture, as well as chemicals. Because of theseproperties, monocomponent silicone masses hardening with the splittingoff of carboxylic acids are preferably used for sealing purposes.

A disadvantage of the described silicone masses is their lowcompressibility so that, in the case of use e.g. as packing cord, highapplication forces on the constructional parts to be sealed arenecessary in order to achieve the desired compactness. For this reason,in technology there are often used foamable elastomers based onpolyurethanes or on noble metal-catalysed, addition cross-linkedsilicone masses of vinyl group-containing siloxanes and hydrogensiloxanes for the production of seals. However, the field of use ofpolyurethane systems is restricted by their limited stability againstthe action of higher temperatures and also by certain chemicalmaterials. Foamable noble metal-, preponderantly platinum-catalysedaddition cross-linking silicone masses, admittedly have a substantiallyhigher temperature stability but have the disadvantage that theseproducts have no or only a very small inherent adhesion to the materialsto be sealed. Furthermore, in the reactive state, thus before foaming upand hardening, these systems are extremely susceptible to certainchemical materials, especially sulphur- and nitrogen-containingcompounds which, already in the cases of traces, inhibit the catalystsystem of these products and can thus suppress their foaming up andhardening. An application of addition cross-linking silicone foamsystems to materials which contain such catalyst poisons is thus notpossible (cf. EP 0 416 229-A2 and EP 0 416 516-A2).

On the other hand, the initially mentioned acyloxysilane cross-linkingsilicone masses display a sufficient stability at higher temperatures ofuse and in the case of chemical stressing. The cross-linking system ofthese products is insensitive towards sulphur- and nitrogen-containingcompounds. Furthermore, acyloxysilane cross-linking silicones displayinherent adhesion to many usual, especially silicate materials.

However, the hardening of these monocomponent polysiloxane mixturescross-linking at room temperature with take up of moisture takes placecomparatively slowly since the water necessary for the reaction mustdiffuse in gaseous form from the surrounding atmosphere into theinterior of the mass. Therefore, the speed of the hardening throughdecreases continuously with progressing reaction into the interior ofthe mass. In the case of low moisture of the surrounding atmosphere orin the case of an unfavourable ratio of surface to volume of thesilicone mass, the reaction can become very slow or, as in vapour-tightsealed off rooms, can also come to a complete stop. Because of this onlyslow hardening, atmospheric moisture cross-linkingacyloxysilane-containing silicone masses cannot be foamed with knownprocedures, such as e.g. by mixing with propellant gases. The resultingfoam would collapse within a short time. The acceleration of thehardening by addition of liquid water is admittedly mentioned in U.S.Pat. No. 3,133,891 but is not practicable because of the difficultieswith a homogeneous distribution.

The task forming the basis of the invention thus consists in the makingavailable of a process for the foaming of silicone masses based onacyloxysilane cross-linking polysiloxane mixtures, whereby these are toharden within a short time, i.e. within a few minutes, with foaming inorder to prevent a collapsing of the foamed up material. The typicallyadvantageous characteristics of the previously known vulcanisatesarising in the case of atmospheric moisture cross-linking, such as forexample inherent adhesion and stability, are thereby to be substantiallyretained.

The task is solved in that, to the initially mentioned and as such knownmonocomponent, acyloxysilane cross-linking silicone masses, is added,immediately before their use, a hydrogen carbonate as propellant andcross-linking agent. Ammonium hydrogen carbonate is thereby preferred,sodium or potassium carbonate or alkylamine hydrogen carbonates can alsobe added.

The acyloxysilane cross-linking silicone masses usable for the foamingare characterised in that they contain at least the followingcomponents.

A) 100 parts by wt. of an at least bifunctional diorganopolysiloxaneterminated with silanol groups, whereby this is built up from a linearor branched chain of repeating units of the formula I ##STR1## Therehereby signify: R¹ : saturated or unsaturated hydrocarbon radicals with1-10 C-atoms, possibly substituted with halogen or cyano groups, wherebythe radicals R¹ within the polymer chain can be of differentconstruction,

n: a whole number of 1,500 to 10,000,

B) 2 to 20 parts by wt. of an acyloxysilane cross-linker of the generalformula II

    R.sup.1.sub.y --Si--(OCOR.sup.2).sub.4-y (with y=0 and 1)  (II)

R² : monovalent saturated hydrocarbon radical with 1 to 15 carbon atoms.

C) 0 to 100 parts by wt. of highly dispersed or precipitated silicicacids, the specific surface of which according to BET can lie in therange of 40 to 150 m² /g.

As example for the radical R¹ of the component A are to be mentioned anydesired alkyl radicals, such as methyl, ethyl, n-propyl, isopropyl,octyl, dodecyl, octadecyl, but also cyclic, such as cyclopentyl andcyclohexyl. Furthermore, there can be used unsaturated aliphatic andcycloaliphatic radicals, such as vinyl, allyl, cyclopentenyl orcyclohexenyl, and also aromatic, such as phenyl or naphthyl, andaliphatic-substituted aryl radicals, for example benzyl or toluyl.Within a polysiloxane, the radicals R¹ can be the same or different. Itis also possible to mix branched and unbranched polysiloxanes with theabove-described construction and in different chain length. Preferably,there are used polysiloxanes terminated with hydroxyl groups, so-calledα,Ω-dihydroxydiorganopolysiloxanes with methyl and phenyl radicals.

The said radicals can also be used in halogen- and cyano-substitutedform. Examples herefor are 1,1,1-trifluorotoluyl, β-cyanoethyl or o-, m-or p-chlorophenyl radicals.

The viscosity of the diorganopolysiloxanes preferably lies in the rangeof 6000 to 350000 mPas but can also lie outside of this range, forexample when additional chain-lengthening agents and cross-linkers arecontained.

In component B, additional compounds of the formula III

    R.sup.1.sub.2 --Si--(COOR.sup.2).sub.2 or (R.sup.1 0).sub.2 --Si--(COOR.sup.2).sub.2

can be used. These serve--preferably in combination with short-chaineddihydroxydiorganopolysiloxanes--for the lengthening of the chains of thepolysiloxanes used.

Instead of bringing together the components A and the components of theformula III with the mixing with components B, the reaction product ofthe two can also be previously produced and used directly.

To the mixtures of the components A to C can be added further materialsfor the achievement of special properties. To be mentioned here areespecially colouring pigments and soluble dyestuffs, stabilisers againstoxidation and the action of heat, dispersers, reaction catalysts (e.g.organotin compounds, titanium or zirconium esters), fungicides, bondingagents, solvents, flare-protective agents, plasticisers (preferablysilicone oils but also based on hydrocarbons), strengthening fillingagents, such as for example graphite or carbon black, as well as passivefilling agents, such as e.g. calcium carbonate, silicates, quartz meal,glass and carbon fibres, diatomaceous earth, metal powder, as well assynthetic material powder.

Mixtures of the components A to C are storage-stable so long as acontact with moisture and alkalis is excluded.

For the foaming up and cross-linking acceleration, to theseacyloxysilane-containing monocomponent silicone masses is added ahydrogen carbonate. The added amount of hydrogen carbonate preferablyamounts to 1 to 1.3 mole per mol of acyloxy groups present in thesilicone mass to be foamed. In principle, however, this amount can alsobe gone below or exceeded in order to influence the bubble picture, thedensity and the hardening time of the resulting foam and in order toadapt the total system to possibly given requirements or workingparameters. In the case of underdosing, foams result of higher densitywith retarded hardening through. On the other hand, an overdosing leadsto quicker foaming and hardening products with lower foam density. Ascation in the hydrogen carbonate, ammonia is preferred, other avines,sodium or potassium are, however, also suitable.

Mixtures of the components A to C with hydrogen carbonate are notstorage-stable. Therefore, the hydrogen carbonate necessary for thefoaming and for the reaction acceleration is admixed in a suitable formwith the mixture of the components A, B and C immediately before use,preferably, for the better mixing, pasted in silicone oils or polymersof type A.

For the reaction acceleration and foaming, there can be used a hydrogencarbonate of commercially usual quality, whereby this salt is to bepresent in the finest possible distribution, preferably with an averagegrain size of below 50 μm. It has proved to be advantageous topredisperse e.g. crystalline ammonium hydrogen carbonate in silicone oilor silicone polymer according to component A and further to homogenisethis dispersion via a cylinder mill until the desired grain fineness isreached. For the avoidance of sedimentation phenomena during a possiblydesired storage, additional highly dispersed silicic acid can be addedto this pasting.

By means of this procedure, the silicone mass initially consisting ofthe components A, B and C can be admixed to the base components of atwo-component silicone foam, the second component of which contains ahydrogen carbonate and immediately before use of the base components. Inthe case of technical uses, this working up preferably takes place viacommercially usual two component mixing and dosing plant by means ofstatic or dynamic mixing principle.

The mixtures resulting according to the above described process foam andsolidify at temperatures of 20° within 10 to 20 minutes to give a softelastic foam with fine, uniform cell structure. In the case of use of 5%methyl triacetoxysilane in the silicone mixture to be foamed and of amole number of ammonium hydrogen carbonate corresponding to the molenumber of acetoxy groups, the achievable volume increase lies at about100-200%. By increase of the proportion of acetoxysilane and ammoniumhydrogen carbonate in the total mixture, the volume increase can beincreased.

If the mixing of the foam components takes place at increasedtemperatures (up to 70° C.), then, as is to be expected, this leads to adistinctly quicker hardening and to greater volume increase.

The foams according to the invention adhere inherently to substrates ofglass, ceramic, wood, pigments and lacquers, concrete, plaster, metalsand synthetic materials. Therefore, they are advantageously used assealing foams on the above-mentioned materials but are also suitable asprotective coatings for electrical or thermal insulation, for vibrationreduction and also as form masses for the production of impressions orof other formed parts which are suitably produced from foamedelastomers.

In the following, the invention is explained in more detail on the basisof an example:

100 parts by wt. of a component I consisting of

60.55 parts by wt. of an α,Ω-dihydroxydimethylpoly-siloxane with aviscosity of 20,000 mPas,

5.55 parts by wt. ethyl triacetoxysilane,

2.60 parts by wt. of a highly dispersed silicic acid with a specificsurface according to BET of about 50 m² /g,

15.10 parts by wt. of an iron oxide pigment,

15.10 parts by wt. of a quartz meal,

0.01 part by wt. dibutyl tin dilaurate, are homogeneously mixed togetherat room temperature with 20 parts by wt. of a component II consisting of72 parts by wt. of an α,Ω-dihydroxydimethylpoly-siloxane with aviscosity of 6,000 mPas, 3 parts by wt. of a highly dispersed silicicacid with a specific surface according to BET of about 110 m² /g, 25parts by wt. ammonium hydrogen carbonate with a grain fineness of <50μm.

The mixture obtained of the components foams at temperatures of 20° C.within 20 minutes to give a mechanically loadable elastomer foam withuniform cell structure. The density of the foam lies at 0.6 g/cm³. Inthe case of the silicone base components used, during this time there isshown only a slight skin formation due to cross-linking because of themoisture of the surrounding atmosphere.

If one brings the mixed foam components on to surfaces of glass, glassceramic, enamel or porcelain, then the resulting foam binds with thesematerials so that it can only be removed from the surfaces by mechanicaldestruction.

If one stores samples of the resulting foam for 50 days at a temperatureof 250° C., then, in the case of the vulcanisate, there is a weight lossof 10%. The strength and ductility of the foam is hereby maintained.

I claim:
 1. A process for foaming a mixture of a bifunctionallyterminated diorganopolysiloxane and a acyloxysilane cross-linking agent,comprising adding to the mixture a sufficient amount of a hydrogencarbonate compound to neutralize the acid in a foam composition, whereinthe hydrogen carbonate compound has a cation portion comprising an aminonitrogen bearing entirely hydrogen substituents or at least one alkylsubstituent.
 2. A process according to claim 1, wherein the hydrogencarbonate compound is present as powder with a grain size of below 50 μmand is pre-suspended with the diorganopolysiloxane or a silicone oil. 3.A process according to claim 1, further comprising adding to thepolysiloxane mixture and/or to the hydrogen carbonate compound a fillingmaterial.
 4. A process according to claim 1, wherein thediorganopolysiloxane is a compound of formula I ##STR2## in which R¹ aresaturated or unsaturated hydrocarbon radicals with 1-10 C-atoms,optionally substituted with halogen or cyano groups, wherein theradicals R¹ within the polymer chain can be of different construction,andn is a whole number from 1500 to 10,000 and, said cross-linking agentcomprises a compound of formula II

    R.sup.1.sub.y --Si--(OCOR.sup.2).sub.4-y (with y=0 and 1)  (II)

whereinR² is a monovalent saturated hydrocarbon radical with 1 to 15carbon atoms.
 5. A process according to claim 4, wherein the compound offormula I has a viscosity of 6,000 to 350,000 mPas.
 6. A processaccording to claim 1, further comprising adding to the mixture at leastone selected from pigments and soluble dyestuffs, stabilisers againstoxidation and the action of heat, dispersers, reaction catalysts,fungicides, adhesives, solvents, flame protection agents, plasticisers,and filling materials.
 7. A silicone foam produced by a processaccording to claim
 1. 8. A method of treating surfaces using thesilicone foam of according to claim 7 comprising coating a substrateselected from glass, ceramic, wood, pigments, lacquers, concrete,plaster, metals, synthetic materials and sealing and formed masses.
 9. Aprocess according to claim 3, wherein the filling material is highlydispersed silicic acid.
 10. A process according to claim 6, wherein thestrengthening filling materials are graphite or carbon black.
 11. Aprocess according to claim 6, wherein the filling materials are passivefilling materials selected from the group consisting of calciumcarbonate, silicates, quartz meal, glass and carbon fibers, diatomaceousearth, metal powder, and metal oxides.
 12. A process according to claim1, wherein the hydrogen carbonate compound is ammonium hydrogencarbonate.
 13. A process according to claim 1, wherein thediorganopolysiloxane comprises α,Ω-dihydroxydimethylpolysiloxane, andthe acyloxysilane cross-linking agent comprises ethyl triacetoxysilane.